Update transform-es2015 READMEs from babel.github.io [skip ci] (#4926)

Signed-off-by: Brian Ng <bng412@gmail.com>

packages/babel-plugin-transform-es2015-classes/README.md

@@ -1,6 +1,12 @@
 # babel-plugin-transform-es2015-classes
 
-Compile ES2015 classes to ES5
+> Compile ES2015 classes to ES5
+
+## Caveats
+
+Built-in classes such as `Date`, `Array`, `DOM` etc cannot be properly subclassed
+due to limitations in ES5 (for the [es2015-classes](/docs/plugins/transform-es2015-classes) plugin).
+You can try to use [babel-plugin-transform-builtin-extend](https://github.com/loganfsmyth/babel-plugin-transform-builtin-extend) based on `Object.setPrototypeOf` and `Reflect.construct`, but it also has some limitations.
 
 ## Installation
 
@@ -43,3 +49,55 @@ require("babel-core").transform("code", {
   plugins: ["transform-es2015-classes"]
 });
 ```
+
+## Options `loose`
+
+#### Method enumerability
+
+Please note that in loose mode class methods **are** enumerable. This is not in line
+with the spec and you may run into issues.
+
+#### Method assignment
+
+Under loose mode, methods are defined on the class prototype with simple assignments
+instead of being defined. This can result in the following not working:
+
+```javascript
+class Foo {
+  set bar() {
+    throw new Error("foo!");
+  }
+}
+
+class Bar extends Foo {
+  bar() {
+    // will throw an error when this method is defined
+  }
+}
+```
+
+When `Bar.prototype.foo` is defined it triggers the setter on `Foo`. This is a
+case that is very unlikely to appear in production code however it's something
+to keep in mind.
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```js
+// without options
+{
+  "plugins": ["transform-es2015-classes"]
+}
+
+// with options
+{
+  "plugins": [
+    ["transform-es2015-classes", {
+      "loose": true
+    }]
+  ]
+}
+```